This invention relates to jet drop recording devices, and more particularly to the problems associated with terminating operation of the printing head in such devices upon completion of the desired amount of printing.
These jet drop recording devices utilize a liquid coating material, such as ink, which is pumped under pressure to a manifold communicating with a series of small diameter orifices as taught by Beam, U.S. Pat. No. 3,577,198, and Mathis, U.S. Pat. No. 3,701,998. As the coating material is ejected through the orifices under pressure, it forms fine filaments of coating material which then break down into a series of discrete drops. At the point where the drops break from the filaments they pass through charging electrodes which, depending on the pattern of coating material desired on a receiving member conveyed beneath the drop generator, either charge or do not charge each drop of coating material.
An electrostatic deflecting field is set up downstream of the charge rings and all drops which receive a charge while passing through the charge rings are deflected from their trajectory by the deflecting field. A catcher is also associated with the system to catch those drops which it is desired to prevent from reaching the receiving member. Thus, by applying suitable charging signals to the drops, a visible, human readable record may be formed on the recording medium.
As can be appreciated, the problem of splattering of the coating material on system components is critical because multi-jet ink recording systems of this type require that the streams of drops be closely spaced. This in turn requires closely spaced orifices and closely spaced charge electrodes. With such closely spaced drop streams, it is difficult to maintain the closely spaced system components free of ink, which is electrically conductive and can cause shorting and other electrical problems. Additionally, evaporation of the ink leaves a residue on the system components which may build up and eventually affect operation.
These problems are especially difficult to remedy during startup and shutdown of jet drop recording devices. For example, if during startup the flow of ink to the drop generator is commenced by simply opening a supply line to the manifold, it will require a finite time period before normal operating pressure is reached. During that time, ink will initially weep from each orifice and form pendulous masses of material along the bottom of the orifice plate. As pressure increases inside the manifold, uncontrolled jets of ink will finally be expelled from the orifices, stabilizing only after the excess ink along the underside of the orifice plate is carried away by entrainment with the jets.
At shutdown, the opposite problem occurs. If the ink supply to the manifold is merely turned off, pressure will gradually decrease collapsing the flowing jets of ink and concluding with the dribbling of ink from the orifices. Obviously, ink spattering will occur which can cause electrical shorts, burnouts, and residue buildup on parts. Various efforts have been made in the prior art to solve this shutdown problem. Examples are Culp, U.S. Pat. No. 3,618,858, who teaches use of open-sided charge electrodes which are moved out of the way during startup and shutdown, and Stoneburner, U.S. Pat. No. 3,891,121, and Perry et al, U.S. Pat. No. 4,042,937, who teach simultaneously closing the fluid supply inlet valve to the ink manifold and opening an outlet valve from the manifold to a vacuum source.
However, none of the prior art shutdown methods have proved entirely satisfactory. Use of movable charge electrodes adds to the complexity of the recording device, while closing the fluid supply inlet and simultaneously opening the ink supply manifold to a vacuum source results in pressure oscillations in the manifold and tends to cause air and other contaminants to be sucked into the ink supply. This necessitates the addition of purging procedures to remove the air and contaminants at a later time.
The problem of achieving clean shutdown is especially acute in the office copier environment where ink is maintained in the manifold at all times to permit rapid startup of the jet recording device. The necessity of a purging or cleaning procedure prior to each startup greatly increases the startup time for each copying cycle and is highly undesirable. Accordingly, the need still exists in the art for a simple shutdown procedure which will prevent splattering of ink and yet keep out air and other conteminants from the ink supply.